Ontogeny and oestradiol dependence of vitamin D-binding protein blood levels in chickens.

The concentration of 25-hydroxyvitamin D3-binding protein (DBP) was measured, by immunodiffusion, in the blood of chickens from embryonic stages to sexual maturity. Low levels of DBP and 1,25-(OH)2D3 were detectable in the blood of chick embryos from the 12th and 17th day of incubation respectively and stayed at the same low levels until hatching. The blood concentration of DBP doubled between the 1st and 5th days of life, then increased slowly and reached the mean level of the adult male at 7-8 weeks of age. The concentration of DBP was independent of vitamin D status in growing chickens. A large increase was observed in DBP blood levels in hens just before sexual maturity. This change, and those observed in moulting hens, followed the variations in plasma concentrations of oestradiol more closely than those of progesterone or testosterone. Moreover, a large increase in plasma DBP levels was induced in immature chickens by oestradiol (0.5 mg/day), but not by testosterone or progesterone. Finally, the experimental suppression of egg shell formation and the associated decrease in 1,25-(OH)2D3 plasma levels had no effect on plasma DBP concentrations. However, 1,25-(OH)2D3 and DBP levels were higher in hens laying shell-less eggs than in immature pullets. The increases in DBP levels at hatching, in immature pullets treated with oestrogens, in hens laying uncalcified eggs and at the onset of egg production were associated with increases in 1,25-(OH)2D3, suggesting a relationship between the levels of DBP and 1,25-(OH)2D3 in the blood.